1. Field of the Invention
The subject invention relates generally to the assembly of gate valves and more particularly to an apparatus and method for compressing packing rings in a stuffing box of a valve bonnet to provide even distribution of pressure between the packing rings and valve stem and therefore a longer packing ring life.
2. Background of the Related Art
Mechanical valves are used to start, stop, and regulate the flow of media through a pipe line or vessel. Valves accomplish this feat by way of allowing for the positioning of a mechanical "obstruction" (e.g., gate, globe, plug, ball, wedge, disc, diaphragm, check, etc.) in the pathway of the media flowing through the valve. The position of this "obstruction" (hereafter referred to as a gate) is controlled by its attachment to one end of an adjustable shaft or valve stem. The other end of this valve stem penetrates through a portion of the valve's body, called the valve bonnet, to the outside environment. A set of packing rings establish a seal between the valve stem and the valve bonnet; thereby, preventing the media in the valve from escaping to the outside environment. These rings also permit the stem to be actuated through its reciprocating or rotary functional length so the gate may be moved between the open and closed positions.
The packing rings are seated within a stuffing box formed in the valve bonnet. The stuffing box includes an annular cavity that surrounds the valve stem at its penetration point into the valve bonnet. A gland follower having a flange supporting an annular collar encircles the valve shaft and encloses the opening to the stuffing box, capturing the packing rings therebetween. Fasteners are used to draw the gland follower toward the valve bonnet compressing the packing rings between the walls of the stuffing box bore and the valve stem. Periodically, the gland follower must be readjusted to compensate for wear seen by the packing rings during their life. Once the gland follower has been adjusted to the acceptable degree of gland follower penetration into the stuffing box cavity, the valve must be taken out of service so the original packing rings can be replaced.
A mechanical actuator such as a hand operated wheel or a remotely operated motor is affixed to the outside end of the valve stem. This device, by its mechanical control of the valve stem, controls the position of the gate and its degree of obstruction to media flow through the valve.
Packing rings can be installed into the stuffing box in a single step procedure by inserting and compressing them as a set; or in a multi-step procedure by inserting and compressing less than all of the rings from a set and then inserting and compressing the complement of the rings from the set. The decision on which procedure to use is heavily dependent upon a number of factors including the type of rings, the dimensions of the stuffing box, the acceptable degree of compressive loading uniformity throughout all of the rings, and whether the benefits of conducting the multi-step installation procedure is justified by the additional expense in carrying it out. For reasons described below, the benefit in conducting the multi-step procedure is that the packing ring life is extended substantially.
The single step installation procedure calls for the ring installation and compression steps to be accomplished in one sequence of events. The stuffing box is loaded with individual packing rings until it is filled, the gland follower is assembled to the valve bonnet, and the fasteners are tightened to exert a compressive loading force against the packing rings. This procedure is the least time consuming because each of the individual installation steps is done only once. However, it does have several major performance drawbacks. For example, because the packing rings fit tightly into the stuffing box bore they are somewhat difficult to insert and are often damaged or at least distorted during the insertion process. Also, as the packing rings are compressed by the gland follower, the upper most packing rings tend to radially expand against the stuffing box bore and the valve stem, more so than the lower packing rings do, thereby causing uneven sealing therebetween. Uneven sealing between the packing rings and the valve stem leads to premature seal failure since the media in the valve tends to work its way up into the packing ring set.
It is well understood by those skilled in the art that uneven compressive loading throughout the packing ring set is caused by frictional forces that are established between the upper rings, and the valve stem and stuffing box bore as the gland follower is drawn into the stuffing box to compress the packing rings. This occurs because the upper rings are compressed by the gland follower and begin to expand sooner than the lower rings. And, because they are compressed sooner, they start to grip the valve stem and stuffing box bore sooner, thereby reducing the amount of axial compressive loading forces that are transmitted to lower packing rings. It is estimated that the resulting differential compressive loading forces between individual packing rings in a stuffing box is as great as about 50%. Therefore, a packing ring that is adjacent to the gland follower will experience the full compressive loading force exerted by the gland follower. However, the second ring (in descending order) will experience only about 50% of the initial axial loading forces, the third ring only about 25%, the fourth ring only about 12.5%, etc.
In order to form a seal that has the maximum degree of sealing effectiveness over its service life span, all of the installed packing rings should be equally compressed. If this is accomplished, all rings contribute equally to sealing. While the single step installation procedure is the most time efficient method of installing packing rings, it guarantees that the packing set will have varying degrees of axial loading and radial sealing forces distributed among its rings.
The conventional multi-step installation procedure calls for the ring insertion and compression steps to be accomplished in several repetitive cycles. The stuffing box is loaded with less than a full set of packing rings, the gland follower is positioned and bolted into place, and the fasteners are tightened to exert a compressive loading force on the packing seals. After loading is applied, the gland follower is removed and the remaining rings from the packing ring set are added to the stuffing box. Thereafter, the gland follower is repositioned and rebolted in place, and the fasteners are again tightened to compress the complete set. Following this procedure it is often found that at least one additional ring needs to be added to the stuffing box because of the space created by the additional compression of the packing set.
Although it would be more ideal to insert and compress each packing ring individually to attain maximum sealing forces throughout the packing ring set, inserting and compressing the packing set in two steps certainly provides a closer approximation to the ideal state than does the single step installation procedure. The major drawback to the multi-step procedure versus the single step procedure is the added time it takes to carry it out. The additional time it takes to perform the multi-step installation procedure adds a substantial amount of cost to each valve bonnet. And as the number of valve bonnets being assembled increases, it takes proportionally more time to assemble each bonnet due to such factors as operator fatigue.
Original equipment manufacturer's (OEM's) of gate valves are under a considerable amount of competitive pressure to keep their manufacturing costs and unit selling prices as low as possible. Very little can be done to significantly reduce the raw material and manufacturing (e.g., casting, machining, etc.) related costs since these are necessary aspects of the base product. Much of the design, metallurgy, wall thickness, part sizes, etc. are controlled by industry standards such as those developed by the American Petroleum Institute. One of the few areas where a significant impact on cost reduction can be influenced relates to the assembly cost of each unit. OEM's are especially sensitive to this cost and place a heavy emphasis on reducing the labor cost per unit of production. As a result, the competitive market influences force OEMs to take a hard look at labor costs in deciding which assembly procedures to follow. Procedures must be chosen that provide an acceptable compromise between optimal installation procedures and maximum throughput.
Clearly there is a need in the art for apparatuses and methods that would enable an OEM to utilize the multi-step installation procedure when installing packing rings into a valve bonnet to improve both the quality of the seal between the stem and valve bonnet and to reduce the amount of time in which the procedure is accomplished. There is also clearly a need for new apparatuses and methods that can further improve the sealing relationship provided by the multi-step installation procedure and do so within the time constraints imposed by the market forces.